Field of the Invention
The present invention pertains to monitoring the thinning of pipes and, more particularly, to a method for monitoring thinning of pipe walls in piping systems to allow repair or replacement of pipes having unduly thinned walls prior to rupture of the pipes.
Discussion of the Prior Art
Many efforts have been made in the past to detect thinning of pipe walls prior to rupture of the pipes; however, such systems have either been ineffective or economically infeasible due to the cost of actually determining wall thickness and/or the downtime required for inspection. Pipe thinning in high pressure piping systems, such as those in the power generating industry, present major safety hazards since rupture or failure of pipes in such systems can cause catastrophic accidents.
Pipe thinning due to erosion/corrosion has become a particular problem in the secondary piping systems on the liquid side of power generating plants since rupture or failure of pipes in such piping systems present great potential for bodily injury and economic injury due to the unscheduled downtime. In one accident in a power generating plant that had been in existence for only one-third of its design life, a pipe elbow ruptured, and measurement of thinning caused by erosion/corrosion revealed that the wall thickness at the time of rupture was 0.01 inches, thinned from a beginning of life thickness of 0.50 inches.
One manner in which to combat the dangerous conditions existing from pipe thinning in the power generating industry is to replace commonly used low carbon steel pipes with pipes made of steel containing more chromium; however, such replacement is economically prohibitive due to the cost of replacement coupled with the downtime of the power generating plant.
In view of the above, it will be appreciated that there is a great need for advanced warning of pipe thinning to permit repair or replacement of unduly thinned pipes during scheduled plant downtime and, or course, prior to rupture. Ultrasonic inspection systems currently employed to detect pipe thinning require a sensor to be placed against the outer wall of pipes to be tested and moved from point to point in areas vulnerable to erosion/corrosion. Such ultrasonic inspection has the disadvantage of requiring periodic access to the piping system necessitating extensive removal of insulation from pipes as well as being expensive from a manpower standpoint and from requiring access into power generator plants. The result is that ultrasonic inspection is limited, economically, to keep costs under control, and many pipes are not entirely inspected but rather are inspected only by selective sampling. Such selective inspection is inherently unreliable as evidenced by a recent accident caused by rupture of a steam generator tube at a power generating plant immediately after the plant was brought to power following periodic inspection, the ruptured tube not being selected for inspection.
Other methods of detecting thinning of walls include drilling wells into a pipe wall from the outer side such that a leak occurs after a predetermined amount of pipe wall thinning, as exemplified by U.S. Pat. No. 1,977,177 to De Florenz and No. 2,083,011 to Ducommun. It has also been known to bore holes entirely through the walls of pipes and insert plugs therein such that wall thickness can be determined by removing the plugs and visually inspecting the walls. Additionally, resistive electrical sensors have been imbedded in walls to provide an electrical indication due to resistance change of wear or erosion of the walls as exemplified by U.S. Pat. No. 3,015,950 to Doctor et al.
The use of tracer materials to indicate wear of mechanical elements is exemplified by U.S. Pat. No. 4,027,157 to Gerve' et al disclosing radioactively marked machine components having "wear fragments" introduced into a lubricant circuit to pass through a measuring chamber surrounding a radiation detector to characterize the components exposed to wear via the detected energy of the gamma quanta for multiple-component measurements; U.S. Pat. No. 2,938,125 to Marak disclosing impregnating the surface of a base material, such as a bearing or a piston, with a radioactive or chemically detectable substance and coating the impregnated surface with a wear resistant material free of the substance such that wear exposes the substance and the substance appears in the lubricant and can be detected, such as with the use of a geiger counter; U.S. Pat. No. 2,658,724 to Arps disclosing capsules of tracer material inserted in teeth of a drill for release into the drill to produce a warning signal indicative of drill wear; U.S. Pat. No. 3,797,896 to Bardach disclosing a bearing lined with radioactive atom dopants of two species for detecting an individual bearing experiencing wear through; and U.S.S.R. Pat. No. 1,004,835 disclosing multiple radio-nuclide marks and loss of activation of the marks to determine the degree of breakdown of the surface of an article. U.S. Pat. No. 2,994,778 to Marsh, No. 3,348,052 to Raifsnider et al and No. 3,678,273 to Lewis are exemplary of the positioning of radioactive, abradable material in a fluid stream to monitor the corrosivity of a fluid.
The detection of fuel pin failures by monitoring cover gas is discussed in an article from the Proceedings of the International Conference on Liquid Metal Technology in Energy Production, May 3-6, 1976, pages 777 through 781 entitled "On Line Radiometric Analysis of FFTF's Cover Gas-Basic Analytical Features" and on pages 494 and 495 of Fast Breeder Reactors by Waltar and Reynolds, Pergamon Press, wherein it is indicated that the fuel assembly containing a failed fuel pin can be determined from unique blends of stable xenon and krypton isotopes injected in the fusion gas plenum of each fuel pipe during fabrication thereof.
The prior art, thus, is cognizant of the need to detect pipe thinning and of the use of tracer materials to detect corrosion or wear of machine parts; however, there exists no economically feasible yet dependable and accurate method to detect or monitor pipe thinning without requiring periodic access to the piping system. More particularly, methods for detecting wear of machine parts with the use of tracer materials have the disadvantages of requiring special machine part fabrication and/or machine disassembly for implementation, none of these methods being designed or suitable for detecting or monitoring thinning of pipe walls.